CN108808085B - Electrolyte for improving heat-resistant uncontrol performance of lithium ion battery - Google Patents

Abstract

The invention discloses an electrolyte for improving the thermal runaway performance of a lithium ion battery, which relates to the technical field of lithium ion batteries, and comprises a combination of trimethyl phosphate, a cyclophosphazene derivative flame retardant, a lithium salt, and a protective agent in a certain proportion . The formulation of the electrolyte of the invention can increase the ion transmission rate and improve the basic electrical performance of the lithium ion battery. The electrolyte is inflammable or non-flammable, which can improve the lightning and safety of the electrolyte, and the battery can protect itself in the event of thermal runaway.

Description

An electrolyte for improving thermal runaway performance of lithium-ion batteries

technical field

The invention relates to the technical field of lithium ion batteries, in particular to an electrolyte for improving the thermal runaway performance of lithium ion batteries.

Background technique

In recent years, due to the increasing requirements for environmental protection and the government's accelerated policy guidance on the development of new energy vehicles, many countries in the world have successively issued bans on the sale of fuel vehicles. Under the continuous stimulation of the incentive policies for new energy vehicles, the market demand has doubled. However, due to various complex application conditions, the lithium-ion battery system has potential safety hazards of explosion and combustion, which largely restricts the development of power lithium-ion batteries. While developing high energy densities to meet the needs of market passenger vehicles, safety is a determining factor for their large-scale use.

During the working process of the lithium-ion battery pack, thermal runaway is the most likely safety problem encountered by the lithium-ion battery during use. The module consists of thousands of battery cells placed in a small closed space, and the cell spacing is small, the heat dissipation effect is poor, and the heat is more likely to accumulate and cause safety accidents. Because the chemical reaction inside the lithium-ion battery is the decomposition of the SEI film, the negative electrode material loses its protection and directly contacts the electrolyte, and the lithium embedded in the carbon negative electrode reacts with the electrolyte and the electrolyte itself oxidizes and decomposes, resulting in A large amount of heat induces the decomposition of LixC6 of the negative electrode, the reaction of LixC6 with the PVDF binder, and the decomposition of the positive electrode material. The large amount of heat and gas released by the above-mentioned related exothermic reactions cause the internal temperature and pressure to further increase, igniting the electrolyte, thereby bringing the danger of combustion and explosion to the battery.

At this stage, the electrolyte usually adopts a non-aqueous system, including an organic solvent electrolyte, which is prepared from high-purity organic solvents, lithium salts, necessary additives and other raw materials in a certain proportion. The electrolyte solvent system is all organic carbonates with low flash point, which are easy to decompose and flammable. When the battery explodes, it often causes violent combustion, which increases the degree of harm of safety accidents. In order to solve this problem and increase the safety performance of lithium batteries, it is urgent to improve the thermal runaway performance of lithium batteries. Therefore, the development of an electrolyte system with good thermal stability and self-flammability, and an electrolyte system that takes into account electrical properties is of great significance for the scope of application and future prospects of lithium batteries.

SUMMARY OF THE INVENTION

Based on the technical problems existing in the background art, the present invention proposes an electrolyte for improving the thermal runaway performance of lithium-ion batteries, which can significantly improve the safety performance of lithium-ion batteries in a state of abuse, and can take into account the electrical energy of the cells. performance.

The present invention is achieved through the following technical solutions:

An electrolyte for improving the thermal runaway performance of a lithium ion battery comprises trimethyl phosphate (TMC), a cyclophosphazene derivative (CPP), a lithium salt and a protective agent. It does not contain conventional carbonate flammable organic solvents, the electrolyte is inflammable or non-flammable, and the battery has a self-protection mechanism in the event of thermal runaway.

Preferably, the cyclophosphazene derivative is one or two or more selected from the compounds shown in the following formula I:

Wherein, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , and R ₆ are each independently one selected from the group consisting of phenoxy, haloalkylphenoxy, haloalkoxy, alkoxy, and alkyl.

Preferably, the number of carbon atoms in the alkyl group in the haloalkylphenoxy group is 1-5, and the number of carbon atoms in the haloalkoxy group, the alkoxy group and the alkyl group is 1-5.

Preferably, the halogen atom is F, Cl or Br.

Preferably, the molar ratio of the trimethyl phosphate and the cyclophosphazene derivative is 2:1.

Preferably, the concentration of the lithium salt in the electrolyte is 1-4 mol/L. The use of high-concentration lithium salts improves the ionic conductivity of the electrolyte and makes up for the lack of electrical properties of the solvent. When the content of lithium salt is less than 1mol/L, although the effect of thermal runaway is obvious, the electrical properties will be affected. When the content is greater than 4 mol/L, the electrolyte is too alkaline, which affects the structural stability of positive and negative electrode materials under long-term cycling.

Preferably, the lithium salt is at least one of LiFSI, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiN(SO ₂ CF ₃ ) ₂ , and LiN(SO ₂ C ₂ F ₅ ) ₂ .

Preferably, the protective agent is at least one of SEI film-forming protective agent fluoroethylene carbonate, fluoropropylene sulfite, fluoroethylene carbonate, and fluoroethylene carbonate (FEC).

The beneficial effects of the present invention are:

The solvent of cyclophosphazene structure has the ability to cut off the free radical chain reaction of combustion, which can not only play the role of conducting lithium ions, but also has the characteristics of being non-flammable, and can build a self-protecting battery when the temperature of the cell is too high. mechanism.

In addition, the nitrogen atom of cyclophosphazene has a large polarity and is easy to coordinate with metal atoms, which improves the wetting ability of the pole piece. Therefore, the impedance between the interfaces is reduced, and the concentration polarization of lithium ions is reduced, thereby reducing the Lithium precipitation probability; in addition, hydrophobic substituents are prone to form intermolecular electrostatic forces such as hydrogen bonds with some functional groups in the SEI film, covering the surface of the SEI film, which is equivalent to forming a physical protective film, which can effectively The reduced solvent is reduced on the negative electrode surface. Lithium salts with weak cation-anion interactions were chosen to provide high ion transport capabilities even at high concentrations. By increasing the concentration of lithium salt to improve the ionic conductivity of the battery electrolyte, the ion transfer rate of the electrolyte at room temperature and low temperature can be improved and the freezing point of the electrolyte can be reduced, thereby improving the low temperature performance of the lithium ion battery. In the presence of a large number of lithium salt anions and cations, lithium ions, solvent cations, and lithium salt anions combine to form a new three-dimensional network structure, which can improve the safety of the cell.

The electrolyte of the present invention uses cyclophosphazene as the main solvent (due to the difference in derivatives, the mass concentration is 50-60%), and cooperates with trimethyl phosphate and high-concentration lithium salt to transmit lithium ions and prevent thermal runaway batteries The effect of combustion.

Detailed ways

In order to better understand the present invention, the present invention will be further described below with reference to the examples, which are only to illustrate the present invention but not to limit it.

Comparative ratio

According to Comparative Example 1 in Table 1, a lithium-ion battery electrolyte containing additives was prepared, and the electrolyte was injected into an aluminum-shell 15Ah lithium iron phosphate system lithium-ion battery. Keep the cell at 150°C, 200°C, and 300°C for 30 minutes to observe whether thermal runaway occurs. The test results are shown in Table 1.

Examples 1 to 6

Lithium-ion battery electrolytes containing additives were prepared according to Examples 1 to 6 in Table 1, and the electrolytes were injected into the lithium-ion battery of 15Ah lithium iron phosphate system with an aluminum shell. Keep the cell at 150°C, 200°C, and 300°C for 30 minutes to observe whether thermal runaway occurs. The test results are shown in Table 1.

Table 1 Electrolyte composition and capacity retention data of Comparative Examples and Examples 1-6

It can be seen from the above Table 1 that the electrolyte prepared by the present invention is injected into the lithium ion battery of the 15Ah lithium iron phosphate system in the aluminum shell, and heated at 300° C. for 30 minutes, and the safety performance is still not affected. Continue to heat up to 400 degrees, 500 degrees, the cells remain intact, and no thermal runaway occurs.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (7)

1. an electrolyte that improves thermal runaway performance of lithium ion battery, is characterized in that: be made up of trimethyl phosphate, cyclophosphazene derivative, lithium salt and protective agent, and described cyclophosphazene derivative is selected from the following One or more of the compounds shown in the formula I:

Wherein, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , and R ₆ are each independently one selected from the group consisting of phenoxy, haloalkylphenoxy, haloalkoxy, alkoxy, and alkyl. 2 . The electrolyte for improving the thermal runaway performance of lithium ion batteries according to claim 1 , wherein the number of carbon atoms of the alkyl group in the haloalkylphenoxy group is 1 to 5, and the haloalkoxy group, The number of carbon atoms in the alkoxy group and the alkyl group is 1 to 5. 3 . The electrolyte for improving the thermal runaway performance of a lithium ion battery according to claim 1 , wherein the halogen atom is F, Cl or Br. 4 . 4 . The electrolyte for improving the thermal runaway performance of a lithium ion battery according to claim 1 , wherein the molar ratio of the trimethyl phosphate and the cyclophosphazene derivative is 2:1. 5 . 5 . The electrolyte solution for improving the thermal runaway performance of a lithium ion battery according to claim 1 , wherein the concentration of the lithium salt in the electrolyte solution is 1-4 mol/L. 6 . 6. The electrolyte for improving the thermal runaway performance of a lithium ion battery according to claim 1 or 5, wherein the lithium salt is LiFSI, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiN ( At least one of SO ₂ CF ₃ ) ₂ and LiN(SO ₂ C ₂ F ₅ ) ₂ . 7. The electrolyte for improving the thermal runaway performance of lithium ion batteries according to claim 1, wherein the protective agent is SEI film-forming protective agent fluoroethylene carbonate, fluoropropylene sulfite , at least one of fluoroethylene carbonate and fluoroethylene carbonate.